Field of the Invention and Related Art
The present invention relates to a release-type dot print head.
Release type dot print heads are well known. In a known release type dot print head, armatures biased in the printing direction by springs are attracted to cores placed respectively in solenoids by the magnetic force of permanent magnets, respectively, each armature is released by energizing the solenoid coil to cancel the magnetic force of the permanent magnet by the that of the solenoid coil so that the armature is moved in the printing direction by the spring to print a dot with a needle joined to the armature for character generation.
There has been strong demand for increasing the printing speed of dot print head regardless of type. To increase the printing speed of the release type dot print head, the force biasing the armature in the printing direction must be increased so that the armature is moved at an increased speed for printing when released, and hence the magnetic flux density in the space between the core and the permanent magnet must be increased accordingly. Japanese Patent Laid-open No. 60-87062 proposes a core and an armature formed of a magnetic alloy containing iron and cobalt as the principal components (Fe-Co magnetic alloy), having a high saturation magnetic flux density to establish a magnetic field of an increased magnetic flux density. An exemplary Fe-Co magnetic alloy contains 49% iron, 49% cobalt and 2% vanadium.
The core and the yoke disclosed in Japanese Patent Laid-open No. 60-87062 are unified in a single core-and-yoke member having a complicated morphology, which is difficult to fabricate and makes making most of the Fe-Co magnetic alloy difficult. When the core-and-yoke member is formed by sintering the Fe-Co magnetic alloy, the density of the core-and-yoke member is comparatively low and hence the saturation magnetic flux density of the same is comparatively low. In forming the core-and-yoke member by precision casting using a lost wax process, the core-and-yoke member is liable to be cracked by a stress induced by shrinkage, and hence the lost wax process is unsuitable. Since the Fe-Co magnetic alloy is brittle it is very difficult to form the core-and-yoke member having a complicated shape by machining. The core-and-yoke member can be formed by electrical discharge machining, in which an ingot of the Fe-Co magnetic alloy is subjected to electrical discharge machining. However, electrical discharge machining increases the manufacturing cost of the core-and-yoke member. Therefore, the core-and-yoke member disclosed in Japanese Patent Laid-open No. 60-87062 has problems in manufacturing process though satisfactory in performance, which makes the practical application of the core-and-yoke member difficult. Practically, the core-and-yoke member is fabricated by a lost wax process using a mixture of the Fe-Co magnetic alloy and an additive for reducing the brittleness of the Fe-Co magnetic alloy. However, the additive reduces the saturation magnetic flux density of the Fe-Co magnetic alloy. Such a core-and-yoke member is hardly satisfactory.
As is mentioned in Japanese Patent Laid-open No. 60-87062, when a plurality of the core-and-yoke members are arranged contiguously, magnetic interference occurs between the adjacent core-and-yoke members, and hence the adjacent core-and-yoke members cannot be simultaneously magnetized. Japanese Patent Laid-open No. 59-114068 discloses individual cores formed of the Fe-Co magnetic alloy and arranged individually on a permanent magnet. Accordingly, magnetic interference occurs hardly between the adjacent cores. However, the cores still have problems in fabricating the same by a lost wax process.